I believe I read someplace that's why they stopped. Can you imagine trying to watch Fantasmic while fireworks are going off behind you? Too distracting.
Well, it wouldn't be much different than having, say, Spectromagic right before Fantasy in the Sky Fireworks. I think they could start the fireworks again, but Fantasmic is such a big show, it can already be considered the big fireworks show at MGM. Mabey MGM should get a nighttime parade?
(Idea: attatch lights to the Disney Stars and Motorcars Parade )